trans-Complementation Rescue of Cyclophilin A-Deficient Viruses Reveals that the Requirement for Cyclophilin A in Human Immunodeficiency Virus Type 1 Replication Is Independent of Its Isomerase Activity

Abstract

Human immunodeficiency virus type 1 (HIV-1) requires the incorporation of cyclophilin A (CypA) for replication. CypA is packaged by binding to the capsid (CA) region of Gag. This interaction is disrupted by cyclosporine (CsA). Preventing CypA incorporation, either by mutations in the binding region of CA or by the presence of CsA, abrogates virus infectivity. Given that CypA possesses an isomerase activity, it has been proposed that CypA acts as an uncoating factor by destabilizing the shell of CA that surrounds the viral genome. However, because the same domain of CypA is responsible for both its isomerase activity and its capacity to be packaged, it has been challenging to determine if isomerase activity is required for HIV-1 replication. To address this issue, we fused CypA to viral protein R (Vpr), creating a Vpr-CypA chimera. Because Vpr is packaged via the p6 region of Gag, this approach bypasses the interaction with CA and allows CypA incorporation even in the presence of CsA. Using this system, we found that Vpr-CypA rescues the infectivity of viruses lacking CypA, either produced in the presence of CsA or mutated in the CypA packaging signal of CA. Furthermore, a Vpr-CypA mutant which has no isomerase activity and no capacity to bind to CA also rescues HIV-1 replication. Thus, this study demonstrates that the isomerase activity of CypA is not required for HIV-1 replication and suggests that the interaction of the catalytic site of CypA with CA serves no other function than to incorporate CypA into viruses.

Cyclophilin A Interacts with HIV-1 Vpr and Is Required for Its Functional Expression

“…A similar situation holds for the CA-CypA interaction, where the prevalent view is that CypA most likely acts as the sole binding partner rather than functioning as a PPIase in CA folding. Even though direct evidence of the latter has been presented recently (57), the incorporation of a catalytically inactive form of CypA into HIV-1 virions that was able to support viral infectivity argues against an isomerase function of CypA in Gag folding (58). In contrast to the CA-CypA interaction, the unusual high content of cis conformers in Vpr argues for a specific requirement of the isomerase function of CypA for Vpr expression.…”

Cyclophilin D, a Component of the Permeability Transition-Pore, Is an Apoptosis Repressor

“…Our finding that cyclophilin D has an activity independent of its peptidylprolyl isomerase activity (Fig. 2C) is not without precedent: cyclophilin A can mediate HIV replication by interacting with Gag without its isomerase function (60), and the porcine kidney peptidylprolyl isomerase can fold the creatine kinase without its catalytic activity (61).…”

Native State Proline Isomerization: An Intrinsic Molecular Switch

“…The interaction between HIV-1 capsid and cyclophilin A is essential for the subsequent packaging of multiple copies of CypA into each HIV-1 virion (37)(38)(39)(40). While the precise function of CypA in HIV virulence is not yet known and the isomerase activity of CypA per se may not be required for HIV replication (41), virions lacking CypA (either by interruption of the interaction with cyclosporin or by mutations in the amino-terminal core domain) are less infectious (39,42).…”